The HI149 monoclonal antibody reacts with human CD1a, a 49kDa protein expressed by cortical thymocytes and dendritic cells including Langerhans cells. The CD1 family of proteins share some structural and functional characteristics with the MHC class I molecules; however, members of the CD1 family are not polymorphic. Similar to MHC class I, CD1a associates with the beta2-microglobulin and is thought to play a role in antigen presentation.